"It seems to be going well," says Timms. "We are genuinely excited about the results we've seen so far."

Chlamydia infection is a major problem for Australia's koala population, causing a kind of conjunctivitis that can lead to blindness, and infections that render female animals sterile.

"As many as 25% to 50% of koalas coming into care in both Queensland and New South Wales are showing clinical signs of the disease and it seems to be getting worse," Timms says.

"When combined with habitat destruction, chlamydial disease continues to be a major threat to koalas' survival," he says.

Modified bacterium

The researchers have been working on a chlamydia vaccine for koalas and humans for several years. Their current formulation uses small genetically engineered molecules from the bacterium responsible for the disease.

The latest study was designed to test whether the vaccine triggered a strong immune response in the animals, and the researchers say they were particularly pleased to see that the shots stimulated immune system T-cells in the vaccinated koalas.

"A good T-cell immune response is essential if the vaccine is to be effective," Timms says. "It suggests we'll get a good degree of protection in these animals."

The researchers are taking a cautious approach in developing the vaccine.

"If all goes well with this trial our future studies will evaluate the vaccine on sick and injured koalas brought in for care, relocated animals, and koalas in other sanctuaries," Timms says.

In the long-term, the results of the koala studies could also help in the development of an effective Chlamydia vaccine for humans, he added.